1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and more particularly to such apparatus provided with an air-fuel ratio feedback control function and a purge control function.
2. Description of the Related Art
It has so far been performed in an internal combustion engine to-absorb an evaporative fuel generated from a fuel tank with activated charcoal and to purge it into an intake system.
Also, there is an internal combustion engine which performs a feedback control of an air-fuel ratio so that in the fuel injection unit the air-fuel ratio of the air-fuel mixture becomes a stoichiometric air-fuel ratio. When, in an internal combustion engine such as this, an evaporative fuel has not been given purge processing, an air-fuel ratio feedback correction coefficient fluctuates with a reference value, for example, 1.0 as center. If the purge processing is started, the air-fuel ratio feedback correction coefficient will assume a less value, because an injection quantity of fuel has to be reduced by the quantity of the purged evaporative fuel.
At the time of this purge processing, a deviation from the reference value of the air-fuel ratio feedback correction coefficient assumes various values, depending upon the operative state of the internal combustion engine, i.e., a ratio of a quantity of purge air to a quantity of intake air (hereinafter referred to as a purge rate). The air-fuel ratio feedback correction coefficient is also set so that it varies relatively slowly with a certain integration coefficient in order to avoid an abrupt variation in the air-fuel ratio. Therefore, when the purge rate varies by a transient operation during purge processing, it takes time for the air-fuel ratio feedback correction coefficient to get from a value obtained before the variation in the purge rate to a value obtained after the variation, and consequently, during this period the air-fuel ratio does not come to be maintained to a stoichiometric air-fuel ratio.
Then, an apparatus such as described below has been proposed in Japanese Patent Laid-Open No. 5-52139.
This internal combustion engine comprises first injection quantity correction means for correcting an injection quantity of fuel with an air-fuel ratio feedback correction coefficient, purge air concentration calculation means for calculating a purge air concentration per target purge rate, based on a shift in the air-fuel ratio feedback correction coefficient which occurs when purge processing is performed, and second injection quantity correction means for reducing a quantity of fuel, based on the product of the purge air concentration and the purge rate when the purge processing is performed. In the internal combustion engine, the maximum purge rate, which is a ratio of a quantity of purge air and a quantity of intake air at the time of the full open state of a purge control valve, is stored in advance, and the duty ratio of the purge control value is set to target purge rate/maximum purge rate so that the target duty ratio is gradually increased when the purge processing is started. When the air-fuel ratio feedback correction coefficient is less than a predetermined value and rich, a purge air concentration coefficient is increased at a constant value by a constant value and also the shift in the air-fuel ratio feedback correction coefficient is reflected in the purge air concentration coefficient at a constant rate at intervals of 15 seconds from the start of the purge processing, thereby forcibly bringing the air-fuel ratio feedback correction coefficient close to 1.0. Thus, the duty ratio of the purge control valve is controlled so that the purge rate becomes constant independently of the operative state of the engine, and even when the purge rate varies, the injection quantity is corrected with the product of the purge rate and the purge air concentration, thereby preventing the shift in the air-fuel ratio at the time of the transition.
However, even if the duty ratio of the purge control valve is controlled so that the purge rate becomes constant and also even if the injection quantity is corrected with the product of the purge rate and the purge air concentration, it takes substantial time to completely calculate the purge air concentration. In other words, it takes substantial time for the air-fuel ratio feedback correction coefficient to become 1.0. For this reason, there is the problem that, until the purge air concentration is completely calculated, the air-fuel ratio cannot be maintained to the stoichiometric air-fuel ratio at the time of the transition from the purge cut state to the purge state, at the time of the transition from the state where the purge rate at the time of an intermediate load can be assured by several percents to the state where the purge rate becomes near 0 as at the time of a high load, or at the time of the return from the high-load state.